Thermally actuated mercury switch



lrxvefitov John L. Slonneger',

- @042 vwm Hi5 Attorney J. L. SLONNEGER THERMALLY ACTUATED MERCURYSWITCH Filed Dec. 29, 1949 Sept. 16, 1952 Patented Sept. 16, 1952THERMALLY ACTUATED MERCURY SWITCH John L. Slonneger, Scotia, N. Y.,assignor to General Electric Company, a corporation of New YorkApplication December 29, 1949, Serial No. 135,753 1 Claim. (01. 200-140)My invention relates to electric switching devices and, moreparticularly, to switching devices which are responsive to temperaturechanges, either increases or decreases, without regard to the exacttemperature at which such changes occur.

Switching devices of the type mentioned are frequently employed incontrol apparatus for oil burning furnaces and the like to indicatewhether or not combustion is established therein. In such a switch aslip-clutch" action is generally employed. For example, the switch maybe actuated in response to a relatively small initial increase intemperature. Following actuation, the switch remains in the actuatedcondition during continuing temperature increases and at steadystatetemperatures established during or following the increase. Subsequently,at an increased temperature the switch may be actuated in the reversedirection in response to a relatively small initial decrease intemperature. Following actuation of the switch the temperature maycontinue to be decreased without further action of switching elementwhich requires relatively little force for the actuation thereof. Inresponse to a relatively small change in temperature in one direction,as for example an increase, actuation of the switch is eifected,following which the switching element is mechanically stopped fromAdditional actuating force further motion. which may be provided byfurther change in temperature of the actuating element is spent inslipping the clutch without moving the switch element. However, shouldthe temperature switch element is immediately actuated in the oppositedirection following which slipping of the clutch again occurs.

It is a principal object of the present inven- .40 change slightly inthe reverse direction the tion to provide a new and improved switchingdevice which is responsive to relatively small changes in temperatureand in which an overtravel action similar to such slip-clutch action isincluded.

In practisin my invention I utilize an expandin which positions fixedelectrodes are contacted in various combinations by the liquid to effectswitching operations. Slip-clutch or overtravel action is obtained byeffecting transfer of the liquid in response to a relatively smallinitial change in gas pressure, following which further change in gaspressure in the same direction results only in bubbling of the gasthrough the liquid. However, a change in gas pressure in the oppositedirection immediately results in a transfer of the liquid to itsoriginal position with subsequent bubbling resulting from further changein gas temperature and pressure.

My invention will be better understood from a consideration of thefollowing description taken in connection with the figures of theaccompanying drawing, and its scope will be pointed out in the appendedclaim. In the drawing, Fig. 1 is a cross-section of a switching deviceembodying my invention; Fig. 2 shows the device of Fig. 1 followingactuation due to an increase of tem perature; and Fig. 3 is across-section of a modi fication of the arrangement shown in Fig. 1.

Referring now to Fig. 1, there is shown a switching device I comprisingprincipally a temperature sensing element 2, a switching element 3 and acapillary tube 4 interconnecting elements 2 and 3. Temperature-sensingelement 2 may conveniently be a metal bulb of a type commonly used inthermostatic apparatus. Bulb 2 is completely sealed except for agas-tight connection 5 to capillary tube 4. Tube 4, which may likewisebe metal, may be relatively long to permit remote location of sensingbulb 2 relative to switching element 3. Switching element 3 is providedwith a housing 6 which is a generally U-shaped member having endportions or legs I and 8 forming the side members of the U, and anintermediate bight portion 9 connecting end portions 1 and 8 and formingthe lower portion of the U. v i

A plurality of switching electrodes H), II and I2 are positioned atpredetermined points in housing 6. More particularly, electrodes l0 and[2 are positioned in legs I and 8 in the vicinity of their junctionswith bight portion 9, while electrode H is positioned at the lowerextremity of bight 9. In this arrangement of electrodes 10, H and 12, asingle-pole, double-throw switch is provided, electrode H being thecommon electrode. It is, of course, necessary for electrodes [0, H and[2 to be insulated from one another. In case housing 6 is made of metal,therefore, it is necessary for electrodes l0 and 12 to be insulated fromhousing 6 by the use of suitable insulating bushings. However, housing,6 :may

conveniently be made of glass, which has the advantage of permitting theinternal conditions of switch element 3 to be viewed. In Fig. 1 housing6 is shown as made of glass or other insulating material and accordinglyno insulating bushings are shown.

Capillary tube 4 ,is-inserted in the outer or free extremity of endportion I and is fixed thereto at a gas-tight joint I3. The free orouter extremity of end portion 8 is provided with a .bulblike portion l4having a predetermined volume and integrally formed with end portion 8.A predetermined quantity of a conducting liquid I5,

preferably mercury, is placed in the lower or bight portion 9 of housing6. The volume of the liquid is such that when the liquid surface ineither end leg I or 8 is at the level of the top wall N5 of the bight 9,the other leg is only partially filled with liquid. The volume of liquidrequiredis such thatwhenliquid l5 stands at equal levels. in legs 8 and'I ,of' housing, 6, contact isnot made with electrodes lil'and l 2. How-.ever under this condition the levelof liquid stands appreciably, above.the top wallfllfi of bight portion 9 between legs 1 and 8. A porousfilter plugfll'l; which may he made of a ceramicmaterial,.is.,positioned in end portion I between the ,end.of capillarytube 4 and electrode I9 and is preferably fixed; to the innersurface ofend portion .1. ...Plug.. IT is not essential to the opera- .tion of theswitch butv may be. included if desired to prevent liquid. I5 fromentering capillary tube land bulb 2 when the apparatus is. inverted, asfor: exampleduring shipment.

Bulbaz, tube 4 andlthe. space in housing 6 not otherwise occupied arecharged. with a gas. I8 which is inertrelative to theliquid and thevari- .ous. materials employed in constructing the elevNo furtheractuation of switch element 3 may occur under such conditions.

When the temperature of bulb 2 ceases to rise, steady-state conditionsare achieved and no further transfer of gas from bulb 2 to bulb I4occurs. Liquid I5 thus remains in the position shown in Fig. 2 andtheswitch remains in the actuated condition shown therein. Under thiscondition the pressure provided by the gas in bulb I4 and column I9equals the pressure in bulb 2. Re-

'ments-ofthe switch. The pressure ofgas. I8

under charging conditions may conveniently be approximately oneatmosphere since such a pressure is'readily obtainable. Followingchargin of theswitch, the entire switching device is sealed .to preventescape of the. gas when the pressure 'thereof'isincreased due toheating.

I In. operation, liquid I5 is first made to assume the positionshown inFig. 1 wherein a. column .I9. ;of liquid I5 is established'in leg Icausing contact ofliquid. I5 with electrode Ill. Column. I3is.-establishedi by tilting switchelement 3 in the direction ofleg I. Inthe tilted. position liquid l5 provides asealbetween the portion of gas.[8a inthe space enclosed by bulb 2, tube 4 and leg 1, andtheportion ofgas l8b in the space enclosed by leg 8- and bulb |4.. For conveniencethese spacesa are referredto hereinafter as bulbs 2 and [4,respectively.- When element 6 is returned to theupright position liquidl5 movesto theposition shown in Fig. 1 causing a slightreduction in thepressure of gas |8a.- and an increase in the pressure of gas I822, thedifference in these; pressures being sufficient; to maintain column .19.

-.-eAsbulb 2. isheated gas I8a therein expands throu h capillary tube 4and filter plug I1 and "frbmesliquid l5v from the position. shown inFig. 1 to thepositionshown in Fig. 2. It willbe 'seen thatthe; positionof Fig. 2 is substantially reversed fromthe. position of Fig. 1 andacolumn trodes. Ill and His interrupted and aconducting'patlrisestablished between electrodes II and gardless of thefinal temperature of bulb 2 and the gas I8a therein, when gas I So iscooled a relatively few degrees contraction thereof occurs, withaccompanying reduced pressure, and liquid I5 is transferred toits-original position as shown in Fig. 1. Further reduction oftemperature of the gas in bulb 2 results in bubbling'of the gas throughliquid 15 without further effect on the position of liquid I5. Dependingon their direction, changes in the temperature of bulb I l and gas I 8?)may assist or oppose normal actuation of theswitch in response totemperature changes of bulb 2,as described. It is desirable, therefore,in

the operation of the switch, thatthe temperature of. bulb, I4 remainsubstantially constant to prevent spurious actuation of the switch.

Itwill be seen that an overtravel or slip-clutch action is. provided byswitch element 3. Liquid I5 may be transferred in either direction by arelatively small change in the temperature of gas [8a in bulb 2 butfurther change of temperature in ,the same direction results in slippingwhich is in the. form of bubbling of gas I8 through liquid Iiwithouteifect on the position of the latter. To efiect actuation of the switchin response to a relatively small change or differential in temperatureit is desirable that the volume of bulb 2 be relatively large withrespect to the total, volume of the space containing gas l8a, this spacecom.- prising bulb 2, tube 4 and leg 1.. Thus thevolumes of tube 4 andleg I are preferably made small relative to the volume of bulb 2. The.volume of the. space enclosed by leg 8 and bulb I4 is preferablyapproximately one-half of the volume of bulb 2; The exact proportionsofthe various elements of the switch may readily be determined in actualpractice .to obtain desired operating characteristics. f

Attention is now directed to Fig. 3 in which thereis shown amodification of the embodiment of my invention previously described.There is shown a. switching device I provided with atemperature-sen'singbulb 2' and a capillary tube .4 which are generally similartocorrespondingelements in Fig. 1. Housing 6'. of switchin element 3',however, differs in construction from housing 6 of switch element 3inFig. 1. Housing 6' is a generally U-shaped member provided with a pairof vertically disposed end portions or legs I and 8. having gas chamberstherein and a bight portionS. 'The latter comprises a relatively heavy-Walled capillary tube 2i interconnecting. the chambers in legs 7' and8'. Bight portion 9. terminates in generally tapered extremities. 22,the larger portions of which are sealed at gas tight joints .23 to legs'I' andB. The reducedportions 25 of tapered extremities 22 extend into.the chambers in legs I and i3. as'shownin Fig. 3. .Thus the-extremities25 of capillary tube 2| are positioned above joints 23, therebyproviding reservoir spaces 26 between tapered portions 24 and the innersurfaces of legs l" and 8, respectively.

As in the device previously described a predetermined quantity of aconducting liquid 15', preferably mercury, is introduced in housing 6,and the space in the system not otherwise cccupied is charged with aninert gas it. Capillary tube 4 preferably extends a substantial distanceinto leg to minimize the possibility of liquid !5' entering capillarytube 4 and bulb 2' when switch I is inverted. The portion of gas [8' inbulb 2, tube l and leg '2" is designated as i812, while the portion ofgas in leg 8 is designated as I82). Housing b is again shown as beingmade of an insulating material such as glass, and accordingly insulatingbushings are not employed in connection with the switch electrodes. Apair of electrodes is and 52' are positioned in leg 1" and 3respectively as in Fig. 1. However, in the arrangement of Fig. 3 a pairof electrodes 21 and 28, rather than a single electrode, such aselectrode H, are provided to act cooperatively with electrodes iii andi2, respectively. Electrodes 27 and 28 are positioned in legs 1 and 3'so as to extend into reservoirs 23 and are provided with a commonconnection 25. It will be seen that regardless of the position of liquid[5, that is, whether the bulk of liquid i5 is in leg 1' or leg 8',portions of liquid it remain permanently in reservoirs 26 since the soleoutlets of legs I and 8 are through extremities 25 of capillary tube 22which are at the upper levels of reservoirs 28. Thus electrodes 21 and28 are in continuous contact with the portions of liquid l5 inreservoirs 26.

Operation of switch I is generally similar to the operation of switch Iin that liquid #5 is forced in response to pressure changes in gas i811.through capillary tube 2! between legs l" and 8. When liquid 55' is inleg 7 as shown in Fig. 3 a conducting path is formed between electrodesl and 2'! thus completing the circuit therebetween. Under this conditionthe circuit between electrodes l2 and 28 is open. When, on the otherhand, liquid i is transferred to leg 8' the reverse is true and acircuit is completed between electrodes l2 and 28 and the circuitbetween electrodes l0 and 21 is open.

The arrangement of Fig. 3 which makes use of a capillary tube 2| tointerconnect legs 1' and 8' is particularly useful to prevent surging ofliquid l5 due to relatively rapid changes in the pressure of gas Ida. Ingeneral liquid I5 is forced completely out of tube 2|, as shown in Fig.3. Regardless of the rate of change of pressure, tube 2| acts as adampening medium to enable gas IE to pass through liquid i5 withoutundue agitation and violence. While the volume of the chamber containinggas l8b is shown as comprisin only leg 8' without a bulb portion similarto bulb l4 in Fig. 1 at the extremity thereof, it will be obvious thatit is only necessary to provide the desired volume therein and that theparticular configuration employed in the construction of housing 6 isprincipally a matter of convenience. It will be further obvious that bysealing the end of leg 1' and by suitably increasing the volume thereof,leg I may be employed as the temperature-sensing element of switch Iwithout the use of bulb 2 and tube 4. In this case it is necessary toprovide thermal insulation between legs '1' and 8', as by lengtheningintermediate portion 8' and capillary tube 2|, to permit changes in thetemperature of leg 7 without appreciably aii'ecting the temperature ofleg 8'.

While I have shown and described a preferred embodiment of my invention,it will be understood that my invention may well take other forms and I,therefore, aim in the appended claim to cover all such changes andmodifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

An electric switch responsive to temperature changes comprising, aU-shaped housing positioned with the leg portions thereof verticallydisposed and with its bight portion at the bottom, a pair of gaschambers forming parts respectively of said leg portions, a capillaryconduit in the said bight portion interconnecting said chambers, a pairof reservoirs within said chambers forming respectively portions thereofwith each reservoir located adjacent the connection between said conduitand the chamber in which it is located, a quantity of electricallyconductive liquid only partially filling said housing but of sufiicientvolume to more than fill both said reservoirs, an inert gas filling saidhousing above said liquid, a pair of electrodes extending through saidhousing respectively into said reservoirs, a second pair of electrodesextending through said housing above said reservoirs respectively insaid two chambers, whereby an upper and a lower electrode are providedin each chamber, a predetermined increase in the pressure of the gas ina first of said chambers relative to the pressure of the gas in thesecond chamber produced by a change in the temperature of the gas in oneof said chambers causing said liquid to flow through said capillaryconduit from said first chamber to said second chamber whereby to raisethe level of the liquid in said second chamber sufficiently to producean electrical connection through said liquid between the upper and lowerelectrodes in said second chamber, and a predetermined inverse change insaid relative pressure produced by an oppositely effective temperaturechange causing said liquid to flow from said second chamber to saidfirst chamber whereby to raise the liquid level in said first chambersufficiently to produce an electrical connection through said liquidbetween the electrodes in said first chamber, said chambers and thereservoirs therein being so constructed and arranged that all Of theliquid in one chamber except that remaining in the reservoir may flowthrough said conduit from said one chamber to the other chamber and thatupon such an occurrence gas flows from said one chamber through saidconduit to the other chamber until the gas pressure in said one chamberis substantially equal to the gas pressure plus liquid head pressure insaid other chamber.

JOHN L. SLONNEGER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,691,016 McCabe et a1 Nov. 6,1928 FOREIGN PATENTS Number Country Date 277,344 Great Britain Dec. 7,1928 407,861 Great Britain Mar. 29, 1934 501,592 Great Britain Feb. 27,1939

